What is the "Full Scale Range" needed to perform a System Gain Calibration on the AD411X family of products?

The discussion of calibration in the datasheets for the various AD411X products is a bit thin. And I'm getting some confusing results with my attempts on an EVAL-AD4111.

So, for the record, what "Full Scale Range" voltage should I place across the SE and Differential inputs for a System Gain calibration?

Does this change if AVDD is 3.3V versus 5.0 volts?

Does it matter if IOVDD is 3.3V or 5V?

Finally, I'm surprised to see offset voltages of hundreds of microvolts after a system offset calibration. Is this to be expected? Note 2 under the specifications states that "Following a system zero-scale calibration, the offset error is in the order of the noise for the programmed output data rate selected." I'm using the SINC3 filter and a 10SPS data rate which on table 8 is in the range of 20uVp-p.

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  • 0
    •  Analog Employees 
    on May 27, 2021 2:22 AM

    Hi, 

    The imitation in the calibration range of the ADC gain for a system full-scale calibration on a voltage input is from 3.75 × VREF to 10.5V x VREF. However,  if 10.5 × VREF is greater than the absolute input voltage specification for the applied AVDD, use the specification as the upper limit instead of 10.5 × VREF (see the Specifications section). 

    With regards to the system offset, how did you perform this? Are you using unipolar or bipolar coding? Are you using a floating input for the Offset calibration or is it an applied 0V input?

    Can you share us other ADC config as well. 

     Thanks,

    Jellenie

  • System Offset is performed by shorting all the inputs ay AVSS

    I'm using unipolar coding

    Other ADC registers are:

    CHANx= enabled with the appropriate MUX selection.

    SETUPx=0x300 (unipolar coding, buffers enabled, external reference)

    FILTERx=0x15 (SINC5 with a SPS setting of 2.5)

    ADCMODE=0x64 (system offset calibration with internal clock)

    IFMODE=0

    Again, I'm doing this on a EVAL-AD4111 attached to my hardware through the SPI signals.

  • +1
    •  Analog Employees 
    on May 28, 2021 2:15 AM in reply to WallyBalls

    Hi, 

    Thanks for sharing these details. Can you try to switch to bipolar mode? 

    When Implementing a system calibration using Unipolar code there is possibility that the Offset calibration will not work correctly.

    For example, if the actual offset was -5mV, then an input signal of 0.0V on a Bipolar encoded ADC would give an ADC Conversion result equivalent to -5mV. Therefore subsequent ADC conversions could be corrected by -5mV in the customer software.

    However, if the ADC is using Unipolar then the ADC result will clamp to code 0x0000. Customer software would think the offset error is 0V, instead of the correct offset value of -5mV. Therefore their software would not adjust future ADC results correctly. 

    And one more thing, the default gain coefficient for AD4111 is calibrated for current inputs. So make sure that you changed it or calibrated it for voltage inputs as specified in the datasheet. 

    Thanks,

    Jellenie

Reply
  • +1
    •  Analog Employees 
    on May 28, 2021 2:15 AM in reply to WallyBalls

    Hi, 

    Thanks for sharing these details. Can you try to switch to bipolar mode? 

    When Implementing a system calibration using Unipolar code there is possibility that the Offset calibration will not work correctly.

    For example, if the actual offset was -5mV, then an input signal of 0.0V on a Bipolar encoded ADC would give an ADC Conversion result equivalent to -5mV. Therefore subsequent ADC conversions could be corrected by -5mV in the customer software.

    However, if the ADC is using Unipolar then the ADC result will clamp to code 0x0000. Customer software would think the offset error is 0V, instead of the correct offset value of -5mV. Therefore their software would not adjust future ADC results correctly. 

    And one more thing, the default gain coefficient for AD4111 is calibrated for current inputs. So make sure that you changed it or calibrated it for voltage inputs as specified in the datasheet. 

    Thanks,

    Jellenie

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